1. Field of the Invention
This invention relates to a method for postweld heat treatment (hereinafter referred to as PWHT) of a welded portion of thick base metal, and more particularly to a method for appropriately judging the point in time for terminating the PWHT in dissipating residual diffusible hydrogen in a welded metal by afterheating.
2. Description of the Prior Art
A problem of cold (or delayed) cracking when welding thick low alloy steels has been known to sometimes occur. At present, it is the usual practice to resort to intermediate stress relief annealing for preventing cracking of that sort. However, as seen in desulfurizing reaction towers and coal liquefaction systems, there is a distinctive trend in recent chemical industries practices toward plants on larger scales, which involve for an increased number of instances of intermediate stress relief annealing after welding. The repetitious intermediate stress relief annealing not only gives rise to deteriorations in strength and toughness of the material but also hinders the fabrication process, inviting increases in the time period for and cost of fabrication.